Planar displays are popular for portable displays and displays with space limits because they are light and small in size. To date, planar displays in addition to liquid crystal displays (LCD), organic electro-luminescent displays (OLED), plasma display panels (PDP) and so on, as well as a mode of the optical interference display are of interest.
U.S. Pat. No. 5,835,255 discloses an array of display units of visible light that can be used in a planar display. Please refer to FIG. 1, which depicts a cross-sectional view of a display unit in the prior art. Every optical interference display unit 100 comprises two walls, 102 and 104. Posts 106 support these two walls 102 and 104, and a cavity 108 is subsequently formed. The distance between these two walls 102 and 104, that is, the length of the cavity 108, is D. One of the walls 102 and 104 is a hemi-transmissible/hemi-reflective layer with an absorption rate that partially absorbs visible light, and the other is a light reflective layer that is deformable when voltage is applied. When the incident light passes through the wall 102 or 104 and arrives in the cavity 108, in all visible light spectra, only the visible light with the wavelength corresponding to the formula 1.1 can generate a constructive interference and can be emitted, that is,2D=Nλ  (1.1)
where N is a natural number.
When the length D of cavity 108 is equal to half of the wavelength times any natural number, a constructive interference is generated and a sharp light wave is emitted. In the meantime, if the observer follows the direction of the incident light, a reflected light with wavelength λ1 can be observed. Therefore, the display unit 100 is “open”.
The first wall 102 is a hemi-transmissible/hemi-reflective electrode that comprises a substrate, an absorption layer, and a dielectric layer. Incident light passing through the first wall 102 is partially absorbed by the absorption layer. The substrate is made from conductive and transparent materials, such as ITO glass or IZO glass. The absorption layer is made from metal, such as aluminum, chromium or silver and so on. The dielectric layer is made from silicon oxide, silicon nitrite or metal oxide. Metal oxide can be obtained by directly oxidizing a portion of the absorption layer. The second wall 104 is a deformable reflective electrode. It shifts up and down by applying a voltage. The second wall 104 is typically made from dielectric materials/conductive transparent materials, or metal/conductive transparent materials.
FIG. 2 depicts a cross-sectional view of a display unit in the prior art after applying a voltage. As shown in FIG. 2, while driven by the voltage, the wall 104 is deformed and falls down towards the wall 102 due to the attraction of static electricity. At this time, the distance between wall 102 and 104, that is, the length of the cavity 108 is not exactly zero, but is d, which can be zero. If we use d instead of D in formula 1.1, only the visible light with a wavelength satisfying formula 1.1, which is λ2, can generate a constructive interference, and be reflected by the wall 104, and pass through the wall 102. Because wall 102 has a high light absorption rate for light with wavelength λ2, all the incident light in the visible light spectrum is filtered out and an observer who follows the direction of the incident light cannot observe any reflected light in the visible light spectrum. The display unit 100 is now “closed”.
Refer to FIG. 1 again, which shows that the posts 106 of the display unit 100 are generally made from negative photoresist materials. Refer to FIGS. 3A to 3C, which depict a method for manufacturing a display unit in the prior art. Referring to FIG. 3A, the first wall 102 and a sacrificial layer 110 are formed in order on a transparent substrate 109, and then an opening 112 is formed in the wall 102 and the sacrificial layer 110. The opening 112 is suitable for forming posts therein. Next, a negative photoresist layer 111 is spin-coated on the sacrificial layer 110 and fills the opening 112. The objective of forming the negative photoresist layer 111 is to form posts between the first wall 102 and the second wall (not shown). A backside exposure process is performed on the negative photoresist layer 111 in the opening 112, in the direction indicated by arrow 113 to the transparent substrate 109. The sacrificial layer 110 must be made from opaque materials, typically metal materials, to meet the needs of the backside exposure process.
Refer to FIG. 3B, which shows that posts 106 remain in the opening 112 after removing the unexposed negative photoresist layer. Then, the wall 104 is formed on the sacrificial layer 110 and posts 106. Referring to FIG. 3C, the sacrificial layer 110 is removed by a release etching process to form a cavity 114. The length D of the cavity 114 is the thickness of the sacrificial layer 110. Therefore, different thicknesses of the sacrificial layers must be used in different processes of the different display units to control reflection of light with different wavelengths.
An array comprising the display unit 100 controlled by voltage operation is sufficient for a single color planar display, but not for a color planar display. A method in the prior art is to manufacture a pixel that comprises three display units with different cavity lengths as shown in FIG. 4, which depicts a cross-sectional view of a matrix color planar display in the prior art. Three display units 302, 304 and 306 are formed as an array on a substrate 300, respectively. Display units 302, 304 and 306 can reflect an incident light 308 to color lights with different wavelengths, for example, which are red, green and blue lights, due to the different lengths of the cavities of the display units 302, 304 and 306. It is not required that different reflective mirrors be used for the display units arranged in the array. More important is that good resolution be provided and the brightness of all color lights is uniform. However, three display units with different lengths of cavities need to be manufactured separately.
Please refer to FIGS. 5A to 5D, which depict cross-sectional views of a method for manufacturing the matrix color planar display in the prior art. In FIG. 5A, the first wall 310 and the first sacrificial layer 312 are formed in order on a transparent substrate 300, and then openings 314, 316, 318, and 320 are formed in the first wall 310 and the sacrificial layer 312 for defining predetermined positions where display units 302, 304, and 306 are formed. The second sacrificial layer 322 is then conformally formed on the first sacrificial layer 312 and in the openings 314, 316, 318, and 320.
Please referring to FIG. 5B, after the second sacrificial layer 322 in and between the openings 314 and 316, and in the openings 318 and 320 is removed by a photolithographic etch process, the third sacrificial layer 324 is conformally formed on the first sacrificial layer 312 and the second sacrificial layer 322 and in the openings 314, 316, 318 and 320.
Please refer to FIG. 5C, which shows that the third sacrificial layer 324 in the openings 318 and 320 remains but the remainder of the third sacrificial layer 324 is removed by a photolithographic etch process. Next, a negative photoresist is spin-coated on the first sacrificial layer 312, the second sacrificial layer 322, and the third sacrificial layer 324, and in the openings 314, 316, 318 and 320, and fills the all openings to form a negative photoresist layer 326. The negative photoresist layer 326 is used for forming posts (not shown) between the first wall 310 and the second wall (not shown).
Please refer to FIG. 5D, which shows that a backside exposure process is performed on the negative photoresist layer 326 in the openings 314, 316, 318 and 320 in a direction of the transparent substrate 300. The sacrificial layer 110 must be made at least from opaque materials, typically metal materials, to meet the needs of the backside exposure process. Posts 328 remain in the openings 314, 316, 318 and 320 after removing the unexposed negative photoresist layer 326. Subsequently, the second wall 330 conformally covers the first sacrificial layer 312, the second sacrificial layer 322, the third sacrificial layer 324 and posts 328.
Afterward, the first sacrificial layer 312, the second sacrificial layer 322, and the third sacrificial layer 324 are removed by a release etching process to form the display units 302, 304, and 306 shown in FIG. 4, wherein the lengths d1, d2, and d3 of three display units 302, 304, and 306 are the thicknesses of the first sacrificial layer 312, the second sacrificial layer 322, and the third sacrificial layer 324, respectively. Therefore, different thicknesses of sacrificial layers must be used in different processes of the different display units, to achieve the objective for controlling reflection of different wavelengths of light.
There are at least three photolithographic etch processes required for manufacturing the matrix color planar display in the prior art, to define the lengths of the cavities of the display units 302, 304, and 306. In order to cooperate with the backside exposure for forming posts, metal materials must be used for making the sacrificial layer. The cost of the complicated manufacturing process is higher, and the yield cannot be increased due to the complicated manufacturing process.
Therefore, it is an important subject to provide a simple method of manufacturing an optical interference display unit structure, for manufacturing a color optical interference display with high resolution, high brightness, simple process and high yield.